Implementing IXmlWriter Part 12: Supporting Pretty-Printing

This is part 12/14 of my Implementing IXmlWriter post series.

Today I will add support for pretty-printing to last time’s IXmlWriter.

Pretty-printing is the addition of whitespace at predetermined locations to make the resulting XML easier to read than when it is all on one line. In the .NET Framework’s System.Xml.XmlTextWriter class, it is supported by the properties Formatting, which allows you to enable or disable pretty-printing; Indentation, which allows you to specify how many whitespace characters indentation should use; and IndentChar, which allows you to specify the whitespace character to use for indentation. For IXmlWriter, I instead chose to expose these features exclusively through the constructor. This frees me from the worry of a user trying to change these properties after IXmlWriter has already begun writing XML, which could produce awkward results. Default parameters are used to make the use of pretty-printing optional and straightforward.

I found the easiest way to implement this feature was to construct a complicated test case and then make the code changes accordingly. However, this method is a little disconcerting due to the lack of building a “gut feeling” of correctness. After finishing the code changes, I did a quick review and convinced myself that the logic seems right, but I doubt I will ever grow the same level of confidence that I would have had I built a mental model of the algorithm first.

In addition to pretty-printing, I also rewrote GetNextNamespacePrefix() to find the first available namespace prefix using brute force by trying ns1:, ns2:, … in order rather than using the relationship between namespace prefixes and the number of namespaces already declared. This should make it easier to add user-defined namespace prefixes in the future, as this feature would break the aforementioned relationship.

Here’s the test case:

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26


StringXmlWriter xmlWriter(StringXmlWriter::Formatting_Indented);
xmlWriter.WriteComment("comment");
xmlWriter.WriteStartElement("root");
  xmlWriter.WriteElementString("child", "value");
  xmlWriter.WriteComment("comment");
  xmlWriter.WriteStartElement("child");
    xmlWriter.WriteAttributeString("att", "value");
  xmlWriter.WriteEndElement();
  xmlWriter.WriteStartElement("child");
    xmlWriter.WriteStartElement("child");
      xmlWriter.WriteStartElement("child");
xmlWriter.WriteEndDocument();

std::string strXML = xmlWriter.GetXmlString();
// strXML should equal (whitespace is important):
// <!--comment-->
// <root>
//   <child>value</child>
//   <!--comment-->
//   <child att="value"/>
//   <child>
//     <child>
//       <child/>
//     </child>
//   </child>
// </root>

Here’s the new header file:

  1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110


// StringXmlWriter.h

class StringXmlWriter
{
public:
    enum Formatting
    {
        Formatting_Indented,
        Formatting_None
    };

    StringXmlWriter(Formatting formatting = Formatting_None,
                    int indentation = 2,
                    char indentChar = " ");

    std::string GetXmlString() const;
    void WriteAttributeString(const std::string& localName,
                              const std::string& text);
    void WriteAttributeString(const std::string& localName,
                              const std::string& ns,
                              const std::string& text);
    void WriteComment(const std::string& text);
    void WriteElementString(const std::string& localName,
                            const std::string& text);
    void WriteElementString(const std::string& localName,
                            const std::string& ns,
                            const std::string& text);
    void WriteEndAttribute();
    void WriteEndDocument();
    void WriteEndElement();
    void WriteStartAttribute(const std::string& localName);
    void WriteStartAttribute(const std::string& localName,
                             const std::string& ns);
    void WriteStartDocument();
    void WriteStartElement(const std::string& localName);
    void WriteStartElement(const std::string& localName,
                           const std::string& ns);
    void WriteString(const std::string& text);

private:
    // PRIVATE TYPES
    // =============
    enum WriteState
    {
        WriteState_Attribute, // An attribute value is being written
        WriteState_Content, // Element content is being written
        WriteState_Element, // An element start tag has been written (and is unclosed)
        WriteState_Prolog, // The prolog is being written
        WriteState_Start, // No Write() methods have been called
    };

    struct OpenElement
    {
        explicit OpenElement(const std::string& localName) :
            QName(localName)
        {
        }

        explicit OpenElement(const std::string& localName,
                             const std::string& prefix) :
            QName(prefix.empty() ? localName : prefix + ":" + localName)
        {
        }

        // The qualified name (namespace prefix-included) of the
        // opened element
        std::string QName;
        // All namespaces declared in this element (maps namespace
        // to namespace prefix)
        typedef std::map<std::string, std::string> Namespaces_t;
        Namespaces_t Namespaces;
    };

    // PRIVATE FUNCTIONS
    // =================
    std::string GetExistingNamespacePrefix(const std::string& ns);
    std::string GetNextNamespacePrefix(const std::string& ns);
    bool NamespacePrefixExists(const std::string& nsPrefix);
    void CloseOpenElement();
    void NewlineAndIndent();

    // PRIVATE MEMBERS
    // ===============
    WriteState m_writeState;

    // Need to use a vector instead of a stack because we must be able
    // to iterate over each opened element in the stack to see if a
    // namespace has already been declared.
    typedef std::vector<OpenElement> OpenedElements_t;
    OpenedElements_t m_openedElements;

    // Needed to track whether content was written inside the XML element
    // so we know how to handle indentation.
    bool m_contentWritten;

    // The style of formatting we are using.
    Formatting m_formatting;

    // The string to use for a single level of indentation.
    std::string m_indentStr;

    // The XML fragment that this class has generated so far. There's no
    // guarantee it will be valid unless WriteEndDocument() is called.
    std::string m_xmlStr;

private:
    // Disable copy construction and assignment
    StringXmlWriter(const StringXmlWriter&);
    StringXmlWriter& operator=(const StringXmlWriter&);
};

Here’s the new implementation file:

  1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447


// StringXmlWriter.cpp

#include "StringXmlWriter.h"

#define ARRAYSIZE(x) ( sizeof(x) / sizeof(x[0]) )

struct CharTranslation
{
    char OriginalChar;
    const char* ReplacementString;
};

static const CharTranslation AttributeValueTranslations[] =
{
    { '"', "&quot;" },
    { '&', "&amp;" },
};

static const CharTranslation CharDataTranslations[] =
{
    { '&', "&amp;" },
    { '<', "&lt;" },
    { '>', "&gt;" },
};

struct OriginalCharEquals :
    public std::binary_function<CharTranslation, char, bool>
{
    bool operator() (const CharTranslation& translation, char ch) const
    {
        return (translation.OriginalChar == ch);
    }
};

static std::string TranslateString(const std::string& originalStr,
                                   const CharTranslation* translations,
                                   int numTranslations)
{
    // Actually one past end, needed for proper std::find_if semantics
    const CharTranslation* endTranslations = translations + numTranslations;

    std::string translatedStr;
    for (std::string::const_iterator stringIter = originalStr.begin();
         stringIter != originalStr.end();
         ++stringIter)
    {
        char ch = *stringIter;
        const CharTranslation* translation = std::find_if
            (
            translations,
            endTranslations,
            std::bind2nd(OriginalCharEquals(), ch)
            );
        if (translation != endTranslations)
        {
            translatedStr += translation->ReplacementString;
        }
        else
        {
            translatedStr += ch;
        }
    }

    return translatedStr;
}

StringXmlWriter::StringXmlWriter(Formatting formatting,
                                 int indentation,
                                 char indentChar) :
    m_writeState(WriteState_Start),
    m_formatting(formatting),
    m_contentWritten(false)
{
    for (int i = 0; i < indentation; ++i)
    {
        m_indentStr += indentChar;
    }
}

std::string StringXmlWriter::GetXmlString() const
{
    return m_xmlStr;
}

void StringXmlWriter::WriteAttributeString(const std::string& localName,
                                           const std::string& text)
{
    WriteStartAttribute(localName);
    WriteString(text);
    WriteEndAttribute();
}

void StringXmlWriter::WriteAttributeString(const std::string& localName,
                                           const std::string& ns,
                                           const std::string& text)
{
    WriteStartAttribute(localName, ns);
    WriteString(text);
    WriteEndAttribute();
}

void StringXmlWriter::WriteComment(const std::string& text)
{
    switch (m_writeState)
    {
    case WriteState_Element:
        // An element is currently open. Close the element so we can open
        // a new one.
        CloseOpenElement();
        // FALL THROUGH
    case WriteState_Content:
    case WriteState_Prolog:
    case WriteState_Start:
        if (m_formatting == Formatting_Indented)
        {
            NewlineAndIndent();
        }
        m_xmlStr += "<!–-";
        m_xmlStr += text;
        m_xmlStr += "–->";
        break;
    default:
        // It doesn't make sense to allow writing comments when writing an
        // attribute value.
        // TODO: Generate error
        break;
    }
}

void StringXmlWriter::WriteElementString(const std::string& localName,
                                         const std::string& text)
{
    WriteStartElement(localName);
    WriteString(text);
    WriteEndElement();
}

void StringXmlWriter::WriteElementString(const std::string& localName,
                                         const std::string& ns,
                                         const std::string& text)
{
    WriteStartElement(localName, ns);
    WriteString(text);
    WriteEndElement();
}

void StringXmlWriter::WriteEndAttribute()
{
    switch (m_writeState)
    {
    case WriteState_Attribute:
        m_xmlStr += '"';
        m_writeState = WriteState_Element;
        break;
    default:
        // TODO: Generate error
        break;
    }
}

void StringXmlWriter::WriteEndDocument()
{
    switch (m_writeState)
    {
    case WriteState_Attribute:
        WriteEndAttribute();
        // FALL THROUGH
    case WriteState_Content:
    case WriteState_Element:
        while (!m_openedElements.empty())
        {
            WriteEndElement();
        }
        break;
    case WriteState_Start:
    case WriteState_Prolog:
        // DO NOTHING
        break;
    default:
        // TODO: Generate error
        break;
    }

    m_writeState = WriteState_Start;
}

void StringXmlWriter::WriteEndElement()
{
    switch (m_writeState)
    {
    case WriteState_Content:
        {
            std::string qname = m_openedElements.back().QName;
            m_openedElements.pop_back();

            if (!m_contentWritten &&
                m_formatting == Formatting_Indented)
            {
                NewlineAndIndent();
            }

            m_xmlStr += "</";
            m_xmlStr += qname;
            m_xmlStr += '>';
            m_writeState = WriteState_Content;
            break;
        }
    case WriteState_Element:
        {
            m_xmlStr += "/>";
            m_openedElements.pop_back();
            m_writeState = WriteState_Content;
            break;
        }
    default:
        // TODO: Generate error
        break;
    }
}

void StringXmlWriter::WriteStartAttribute(const std::string& localName)
{
    WriteStartAttribute(localName, "");
}

void StringXmlWriter::WriteStartAttribute(const std::string& localName,
                                          const std::string& ns)
{
    switch (m_writeState)
    {
    case WriteState_Element:
        {
        std::string nsPrefix;
        bool mustDeclareNamespace = false;

        if (!ns.empty()) {
            nsPrefix = GetExistingNamespacePrefix(ns);
            if (nsPrefix.empty()) {
                nsPrefix = GetNextNamespacePrefix(ns);
                m_openedElements.back().Namespaces[ns] = nsPrefix;
                mustDeclareNamespace = true;
            }
        }

        if (mustDeclareNamespace) {
            m_xmlStr += " xmlns:";
            m_xmlStr += nsPrefix;
            m_xmlStr += "=\"";
            m_xmlStr += ns;
            m_xmlStr += '"';
        }

        m_xmlStr += " ";
        if (!nsPrefix.empty()) {
            m_xmlStr += nsPrefix;
            m_xmlStr += ':';
        }
        m_xmlStr += localName;
        m_xmlStr += "=\"";
        m_writeState = WriteState_Attribute;
        break;
        }
    default:
        // TODO: Generate error
        break;
    }
}

void StringXmlWriter::WriteStartDocument()
{
    switch (m_writeState)
    {
    case WriteState_Start:
        m_xmlStr += "<?xml version=\"1.0\"?>";
        m_writeState = WriteState_Prolog;
        break;
    default:
        // TODO: Generate error
        break;
    }
}

void StringXmlWriter::WriteStartElement(const std::string& localName)
{
    WriteStartElement(localName, "");
}

void StringXmlWriter::WriteStartElement(const std::string& localName,
                                        const std::string& ns)
{
    switch (m_writeState)
    {
    case WriteState_Element:
        // An element is currently open. Close the element so we can open
        // a new one.
        CloseOpenElement();
        // FALL THROUGH
    case WriteState_Content:
    case WriteState_Prolog:
    case WriteState_Start:
        {
        if (m_formatting == Formatting_Indented)
        {
            NewlineAndIndent();
        }

        std::string nsPrefix;
        bool mustDeclareNamespace = false;

        if (!ns.empty()) {
            nsPrefix = GetExistingNamespacePrefix(ns);
            if (nsPrefix.empty()) {
                nsPrefix = GetNextNamespacePrefix(ns);
                mustDeclareNamespace = true;
            }
        }

        OpenElement openElement(localName, nsPrefix);
        if (mustDeclareNamespace) {
            openElement.Namespaces[ns] = nsPrefix;
        }

        m_openedElements.push_back(openElement);

        m_xmlStr += '<';
        if (!nsPrefix.empty()) {
            m_xmlStr += nsPrefix;
            m_xmlStr += ':';
        }
        m_xmlStr += localName;
        if (mustDeclareNamespace) {
            m_xmlStr += " xmlns:";
            m_xmlStr += nsPrefix;
            m_xmlStr += "=\"";
            m_xmlStr += ns;
            m_xmlStr += '"';
        }

        m_writeState = WriteState_Element;
        m_contentWritten = false;
        break;
        }
    default:
        // TODO: Generate error
        break;
    }
}

void StringXmlWriter::WriteString(const std::string& text)
{
    switch (m_writeState)
    {
    case WriteState_Attribute:
        m_xmlStr += TranslateString
            (
            text,
            AttributeValueTranslations,
            ARRAYSIZE(AttributeValueTranslations)
            );
        break;
    case WriteState_Element:
        // An element is currently open. Close the element so we can start
        // writing the element content.
        CloseOpenElement();
        // FALL THROUGH
    case WriteState_Content:
        m_xmlStr += TranslateString
            (
            text,
            CharDataTranslations,
            ARRAYSIZE(CharDataTranslations)
            );
        m_contentWritten = true;
        break;
    default:
        // TODO: Generate error
        break;
    }
}

std::string StringXmlWriter::GetExistingNamespacePrefix(const std::string& ns)
{
    for (OpenedElements_t::const_iterator openElemIter = m_openedElements.begin();
         openElemIter != m_openedElements.end();
         ++openElemIter)
    {
        OpenElement::Namespaces_t::const_iterator nsIter =
            openElemIter->Namespaces.find(ns);
        if (nsIter != openElemIter->Namespaces.end())
        {
            return nsIter->second;
        }
    }

    return "";
}

std::string StringXmlWriter::GetNextNamespacePrefix(const std::string& ns)
{
    std::string nsPrefix;

    for (int i = 1; ; ++i)
    {
        std::stringstream ss;
        ss << "ns" << i;
        std::string nsPrefix = ss.str();
        if (!NamespacePrefixExists(nsPrefix))
            return nsPrefix;
    }
}

bool StringXmlWriter::NamespacePrefixExists(const std::string& nsPrefix)
{
    for (OpenedElements_t::const_iterator iter = m_openedElements.begin();
         iter != m_openedElements.end();
         ++iter)
    {
        for (OpenElement::Namespaces_t::const_iterator nsIter = iter->Namespaces.begin();
             nsIter != iter->Namespaces.end();
             ++nsIter)
        {
            if (nsIter->second == nsPrefix)
                return true;
        }
    }

    return false;
}

void StringXmlWriter::CloseOpenElement()
{
    m_xmlStr += '>';
    m_writeState = WriteState_Content;
}

void StringXmlWriter::NewlineAndIndent()
{
    assert(m_formatting == Formatting_Indented);

    if (!m_xmlStr.empty())
        m_xmlStr += '\n';

    for (int i = 0; i != m_openedElements.size(); ++i)
    {
        m_xmlStr += m_indentStr;
    }
}